1. Field of the Invention
This invention relates to a coin sorting machine utilized for a vending machine, juke box etc., and more particularly to a coin sorting machine in which a sorting coil for detecting the characteristics of a coin inserted thereinto is provided along a coin passage to utilize the variations of the output signal of the sorting coil caused when a coin passes through the position of the sorting coil in order to identify the coin.
2. Description of the Prior Art
A coin sorting machine is known in the art in which the diameter, thickness and weight of a coin is mechanically detected to determine whether it is a true coin or a false coin. In a coin sorting machine of this type, only the diameter, thickness and weight of a coin are inspected regardless of the material of the coin; that is, if the diameter, thickness and weight of a coin are detected as satisfactory or acceptable, then the coin is determined as a true coin. Accordingly, such a machine is very low in coin sorting accuracy and is, therefore, low in reliability.
In order to overcome the above-described drawbacks, a coin sorting machine has been proposed in the art in which, on the basis of the phenomenon that when a coin is moved past a sorting coil connected to an oscillator the impedance of the coil is changed, the sorting coil is provided along the coin passage and the variation of the impedance of the sorting coil caused when a coin passes through the sorting coil is utilized. Heretofore, the following three coin sorting systems employing such a sorting coil are known in the art. A first one is a system in which a bridge circuit is formed with the sorting coil, a reference impedance element compared with the sorting coil, and two other impedance elements, and the balanced state of the bridge circuit is detected when a coin passes through the sorting coil. A second one is a frequency variation detecting system in which an oscillation circuit is formed with the sorting coil as a resonance element, and the variation of the oscillation frequency of the oscillation circuit is detected when a coin passes through the sorting coil. A third one is an induced voltage detecting system in which the sorting coil is formed with an oscillation coil and a reception coil which are opposed to each other, and the variation of the voltage induced in the reception coil is detected when a coin passes between the two coils. These systems are similar to one another in that the coin sorting operation is effected by determining whether or not a sorting signal based on the output of the sorting coil detecting the material, thickness and diameter of a coin inserted into the machine is within a coin discrimination reference range. One example of such a conventional coin sorting machine, that is, a bridge circuit system will now be described with reference to FIG. 1.
Shown in FIG. 1 is a bridge circuit employed for sorting coins in one monetary denomination. The bridge circuit comprises an oscillator Wo, a sorting coil Lo arranged along a coin passage (not shown), a variable resistor R1, a variable coil L1, and fixed resistors r0 and r1. In the bridge circuit, the values of the variable resistor R1 and the variable coil L1 are so adjusted in advance that when a true coin passes along the sorting coil Lo, the bridge's output V1, that is, the voltage between the connection points c and d is made to be zero by the impedance variation of the sorting coil.
The coin sorting operation of the machine using the bridge circuit described above will now be described. If the inserted coin is a true coin, the balance point of the bridge circuit is detected as shown in FIG. 2 illustrating the waveform of the output between the terminals c and d of the bridge circuit. In FIG. 2, the outut V1 of the bridge circuit is plotted on the vertical axis, while the time t related to the speed of a coin rolling along the coin passage is plotted on the horizontal axis. As is apparent from FIG. 2, at the time instant t.sub.1 a coin reaches the position of the sorting coil Lo, as a result of which the impedance of the sorting coil is changed to place the bridge circuit in balanced state. Thus, when a coin passes through the position of the sorting coil Lo, the bridge circuit is balanced only once. This balance point is detected to sort out coins, and it is also utilized as a coin detecting signal. In addition, the system is so designed that even if a coin which is equal in diameter, thickness and weight to a true coin but different in material from the true coin passes through the position of the sorting coil, the bridge circuit is not balanced.
However, when a coin which is equal in material and thickness to the true coin but larger in diameter than the true coin passes through the sorting coil, the bridge circuit is balanced twice as shown in FIG. 3 indicating two balance points. The reason for this phenomenon is that the amount of variation in impedance of the sorting coil Lo caused by the coin larger in diameter is greater than that caused by the true coin. Accordingly, the bridge circuit is balanced once, unbalanced thereafter, and balanced again. More specifically, as the false coin approaches the position of the sorting coil Lo, the impedance thereof is gradually changed. When the impedance is about to reach a value required to balance the bridge circuit, the first balance point e is obtained. Thereafter, the impedance of the sorting coil Lo is further changed, and the bridge circuit is unbalanced. Then, while the false coin is passing through the sorting coil Lo, the impedance of the sorting coil is gradually changed, as a result of which the impedance is about to reach the value required to balance the bridge circuit again, whereupon the second balance point f is obtained. Accordingly, the false coin may be determined as a true coin. Furthermore, the number of coins inserted into the machine may be erroneously counted in the case where the balance point of the bridge circuit is detected twice, and the detection signal is employed as a coin counting signal.
The phenomena described above take place also in the frequency variation detecting system in which the oscillation circuit is formed with the sorting coil as a resonance element, and in the induced voltage detecting system. That is, in these systems also, when a false coin which is made of the same material as that of a true coin but is larger in diameter the true coin passes through the position of the sorting coil, the variation of the oscillation frequency of the voltage in resonance to be detected takes place twice. In these systems, in order to sort the true coin from the false coin, another sorting means is further required, thereby causing the overall mechine to be expensive and the sorting time to be prolonged.